1. Field of the Invention
This invention relates to data processing systems and more particularly to the control of the display hardware of a data processing system.
2. Description of the Related Art
A typical data processing system is made up of layers. One of the layers is an operating system such as an UNIX* operating system, although it could be any other operating system, also. The next layer is called a virtual machine interface which is a logical layer. All of the commands of the operating system that go to devices must go through this virtual machine interface. Below the virtual machine interface layer is a subsystem called a virtual terminal subsystem. The virtual terminal subsystem is responsible for managing all of the hardware devices such as the keyboard, locator, tablet, sound device, and all of the displays, whether in character (alphanumeric) mode or all points addressable (APA) mode. FNT * UNIX is a trademark of AT&T Bell Laboratories.
Generally, alphanumeric displays can only display text characters. However, in all points addressable mode, characters and graphics are supported. For data processing systems that support graphics, the costs are higher than if just text processing is supported.
The following describes the general flow through these layers when a command is sent from an application program through a system to produce output on a display. In order to send a command to display either graphics or characters on the display, the operating system causes a command to go through the virtual machine interface. While in the virtual machine interface layer the command is captured and reformatted into queue elements. From there the command is sent to the virtual terminal subsystem. The virtual terminal subsystem intercepts the command and determines the type of command. Next, the virtual terminal subsystem sends the command to the display.
Since the command to display has to go through all of these layers prior to reaching the display, the system performance is degraded. By going through these layers, the response time of the system is lowered. For example, if an application program utilizes a mouse for cursor movement on the display screen, there may be a significant delay between the mouse movement and the cursor repositioning on the screen of the output display device. Ideally, these two movements should be essentially simultaneous.